1. Field of the Invention
This invention relates to demountable conduction cooled current leads for refrigerated superconducting magents. Such structures of this type generally either allow the warm section of the current lead to be thermally demounted from the magnet after the magnet is powered and placed in persistent mode or both the cold and warm sections of the current leads to be thermally demounted from the magnet after the magnet is powered and placed in persistent mode. In this manner, the head load placed upon the cryocooler is significantly reduced relative to that with the leads connected after the magnet is powered and placed in persistent mode.
2. Description of the Related Art
The implementation of refrigerated magnet technology has the potential to revolutionize superconducting magnet design. Simplification of the cryostat by elimination of the liquid helium vessel and one thermal radiation shield, typically found in a conventional superconducting magnet as well as the elimination of helium refilling or liquefaction, are great advantages of this technology.
One limitation of the technology is the cooling capacity of the cryogenic refrigerator or cryocooler, which is used to cool the magnet cartridge and the thermal shield. Since the temperatures of the cryocooler's first and second stages are, usually, inversely proportional to the heat inputs thereto, it is necessary to maintain those heat inputs below the level at which the magnet and shield temperatures are within their operating ranges. For typically commercially available cryocoolers of the 5 kW size which is optimal for application to an MRI imaging magnet, the first and second stage temperatures vs. heat load may be represented by FIG. 1.
Since the heat loads for a typical magnet of the size required for an MRI scanner are about 33 W at the first stage and 2 W at the second, the shield and magnet operating temperatures are about 40 and 11.5K, respectively. Of these loads, the majority of the second stage load, and about half of the first stage load, can come from the conduction cooled leads. Such leads are required to power the main field windings because of the lack of any boil off helium vapor to cool them. Tables 1-4 below show typical heat loads for a 0.5 Tesla magnet and for a design with the gradient coils integrated into the cryostat as set forth in U.S. patent application Ser. No. 07/757,337. As can be seen, the leads represent a significant portion of the total heat leak, especially at the second stage. T1 TABLE 1-First stage heat inputs for 0.5 Tesla refrigerated magnet? -Radiation 14.3 -Residual gas cond. 1.6 -Conduction 4.4 -Leads 13.2 -Total 33.5 -
TABLE 2 ______________________________________ Second stage heat inputs for 0.5 Tesla refrigerated magnet ______________________________________ Radiation 0.044 Residual gas cond. 0.022 Conduction 0.13 Leads 2.0 Total 2.2 ______________________________________
TABLE 3 ______________________________________ First stage heat inputs for magnet with integrated gradient ______________________________________ coil Radiation 8.0 Residual gas cond. 0.8 Conduction 6.1 Leads 22.5 Generation 26.5 Total 63.9 ______________________________________
TABLE 4 ______________________________________ Second stage heat inputs for magnet with integrated gradient ______________________________________ coil Radiation 0.044 Residual gas cond. 0.022 Conduction 0.13 Leads 2.0 Total 2.2 ______________________________________
While the shield temperature of 40K for the typical magnet is within an acceptable range, the magnet temperature of 11.5K is unsuitable for certain applications because of the limited temperature range of the noibium tin superconducting material usually employed in the field windings. Particularly, magnets which must produce a relatively high field in the bore (&gt;1 T) have a concomitantly high field in the windings (&gt;3 T). The need for some temperature margin, i,e. an operating temperature below the critical temperature, results in an unacceptably low current in a winding at 3 T. By allowing the magnet to operate at a temperature close to its critical value only during the ramping phase of its operation, when the lead heat leak is unavoidable, demountable leads allow a resonable temperature margin during steady state operation.
Another attractive design which is improved by this technology is the placement of the gradient coils inside of the vacuum vessel, so that they operate at the thermal shield temperature of about 40-50 K, generates much less than they would at room temperature, and make the complete magnet/gradient coil package much smaller. In this case, however, the additional head load at the cryocooler first stage from the gradient coil leads and the heat generated in the gradient coils during image sequences results in an unacceptably high first stage temperature or the need for a larger capacity cryocooler. This situation is avoided by use of demountable warm leads, which eliminates 13 of the 22 W of lead heating (the balance being the gradient coil leads, which must remain connected during operation).
The two designs described above are possible if the heat input represented by the conduction cooled leads can be eliminated during routine operation (i.e. after the magnet is ramped up) by the use of such demountable leads. For the high field magnets, the second stage main coil leads would have to be demountable, while for the gradient coil integrated magnet, the first stage leads would have to be demountable. Ideally, both would be designed to be demounted.
Fortunately, MR magnets are typically designed to be operated in a persistent mode, i.e. with a superconducting switch in parallel with the main windings which is heated to a temperature which causes it to be resistive during magnet ramping, and then allowing to cool to a superconducting temperature. The main winding current then flows in a persistent loop through the main windings and the switch, and the current leads may be demounted until the magnet must be depowered. This demountable lead technology is in common use on helium cooled magents.
The realization of a demountable conduction cooled lead is complicated somewhat by the requirement that the cryocooler cold head, which must be thermally attached to the magnet and thermal shield to remove the heat, must at the same time be mechanically decoupled from the magnet and thermal shield. This requirement stems from the vibration induced "motion artifacts" which will appear in images produced by a magnet if it is vibrating during a scan.
Another issue which must be addressed is that if the cryocooler is sized to maintain the magnet and/or thermal shield at the design temperature without the lead heat input(s), the effect of these heat input(s) during magnet ramping must be addressed. Typically, the thermal mass of the magnet cartridge and the thermal shield are sufficient to limit their temperature rises to acceptable levels during ramps of reasonable duration.
It is apparent from the above that there exists a need in the art for a current lead for a refrigerated superconducting magnet which eliminates the use of a liquid helium vessel, and which can be thermally decoupled from the first and possibly also the second stage of the cryocooler once the magnet has been placed in persistent mode. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.